A fundamental approach toward caller location in wireless communication networks is to be able to determine that an emergency service call has originated, to provide the location of the base station that the caller is currently using, and to provide handset/mobile unit ID of the caller.
Though not provided in conventional systems, this information is nonetheless available in certain PCS architectures such as, for example, PACS, formerly called WACS (Bellcore), DECT (European), CDMA (Omniopoint), PHS-PHP (Japan), IS-54 (TDMA), IS-95 (CDMA), PCS-1900 (GSM), and B-CDMA (Oki), and features which may be implemented as private networks or regulated services. PACS architecture provides the capability via a layer 2 protocol for the 911 user's mobile unit to indicate that an emergency call is being attempted, but for which no base station/control unit resources are available. This special signaling is accomplished by allowing the mobile unit to borrow spare "filler bits" which are sent during quiet portions of the air interface signals, in order to squeeze in the emergency request information. Similar means exist in related PCS architecture to provide a lightweight emergency access signal to a processing entity that can further initiate a 911 call and possibly perform location functions.
The prior art thus makes it possible for a mobile unit to indicate to a control unit that an emergency call is being requested, even though no voice time slots (voice communications channels) are available. This approach, while better than sending no information at all, is less than optimal because it fails to provide precise location information since only the location of the originating base station is transmitted. Moreover, the prior art provides no means, other than the use of filler bits, to transmit an emergency call when corresponding base station/control unit resources are busy.
Consequently, a need has developed for a method which will permit more precise position determination and which provides a means for propagating emergency access requests as well as a method for reallocating existing communications in an attempt to free up a voice channel for the transmission of an emergency call.